Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 536

tidal bars is slightly oblique, but in the direction of the dominant current. Migra-
tion proceeds by erosion of their stoss side by the dominant current and depo-
sition on the opposite lee side. This leads to the preferential preservation of the
down-current flank of the bar, where sedimentation is dominated by the subor-
dinate current. As a result, the small cross-beds within the large-scale lateral
accretion deposits are preferentially oriented in the direction of the regional
weaker current ( Dalrymple and Choi, 2007 : fig. 29). The lateral-accretion
deposits of tidal bars typically overlie the erosion surface produced by the
migration of the thalweg of the adjacent channel (e.g., Yu et al., 2012 ). Because
the water depth and the current speed both decrease upward from the thalweg
toward the bar crest, the bedset thicknesses decrease and grain sizes decrease
upward, except where fluid mud occurs at the channel bottom ( Dalrymple
et al., 2003 ). This contrasts with the tidal sand ridges occurring in unconfined
areas of open shelves.
Tidal bars, which typically develop in restricted brackish-water settings (e.g.,
estuaries, funnel-shaped bays), tend to contain low-diversity ichnofaunas domi-
nated by trophic generalists such as Planolites or Teichichnus ( MacEachern and
Gingras, 2007; MacEachern and Pemberton, 1994; M ´ ngano and Buatois, 2004a )
and are addressed by Gingras et al. (2012) .
4. ICHNOFACIES TRENDS
The ichnofacies gradient across tide-dominated shorelines is to some extent
opposite to that observed in wave-dominated shoreface environments. Near
the shoreline, the overall tidal energy increases from the supratidal to the shal-
low subtidal. Whereas the Skolithos Ichnofacies tends to dominate lower inter-
tidal and shallow subtidal deposits, the Cruziana Ichnofacies is best developed
in middle and upper intertidal environments, as well as in low-energy areas
of the inner shelf ( Desjardins et al., 2012a; M´ngano and Buatois, 2004b;
M´ngano et al., 2002a ). The decrease in energy toward the upper intertidal is
accompanied by a decrease in sand content, sediment oxygen content, and
mobility of the substrate. Thus, muds and organic particles are prone to settle
from suspension as the energy decreases toward the upper intertidal zone. How-
ever, the highest concentration (mass per unit volume) of mud and organic par-
ticles occurs in the zone of mixed sediments (muddy sand-sandy mud),
although the highest contents (mass per unit mass) of mud and organics occurs
in pure mud in the uppermost intertidal area ( Flemming, 2012 ). Accordingly,
the highest trace-fossil diversity tends to occur in the mixed-flat area or, in
the case of lower-energy systems, in the sand flat, with ichnofaunas belonging
to the Cruziana Ichnofacies. The decrease in suspended organic particles toward
the upper intertidal is a consequence of the reduction in the thickness of water
column toward the high-tide line (i.e., the total amount of suspended material
available for sedimentation gets progressively smaller; Flemming, 2012 ).